<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <title>Client Hints Infrastructure</title>
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
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/*
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Possible extra rowspan handling

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*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
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		/* Larger, more consistently-sized click target */
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/** Index *********************************************************************/

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/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
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	/* The link in the first column in the property table (formerly a TD) */
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/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

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/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
	.hide { display: none }



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
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  <link href="https://www.w3.org/StyleSheets/TR/2016/cg-draft" rel="stylesheet">
  <link href="https://wicg.github.io/client-hints-infrastructure" rel="canonical">
<style>
table, th, td { border: 1px black solid; }
thead {background-color: yellow; }
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<style>/* style-md-lists */

/* This is a weird hack for me not yet following the commonmark spec
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">Client Hints Infrastructure</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Draft Community Group Report, <time class="dt-updated" datetime="1970-01-01">1 January 1970</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wicg.github.io/client-hints-infrastructure">https://wicg.github.io/client-hints-infrastructure</a>
     <dt class="editor">Editor:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="https://blog.yoav.ws">Yoav Weiss</a> (<span class="p-org org">Google</span>) <a class="u-email email" href="mailto:yoav@yoav.ws">yoav@yoav.ws</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 1970 the Contributors to the Client Hints Infrastructure Specification, published by the <a href="https://www.w3.org/community/wicg/">Web Platform Incubator Community Group</a> under the <a href="https://www.w3.org/community/about/agreements/cla/">W3C Community Contributor License Agreement (CLA)</a>.
A human-readable <a href="http://www.w3.org/community/about/agreements/cla-deed/">summary</a> is available. </p>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Specification of the Client Hints infrastructure and its integration with Fetch and HTML</p>
  </div>
  <div data-fill-with="at-risk"></div>
  <h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
  <div data-fill-with="status">
   <p> This specification was published by the <a href="https://www.w3.org/community/wicg/">Web Platform Incubator Community Group</a>.
  It is not a W3C Standard nor is it on the W3C Standards Track.

  Please note that under the <a href="https://www.w3.org/community/about/agreements/cla/">W3C Community Contributor License Agreement (CLA)</a> there is a limited opt-out and other conditions apply.

  Learn more about <a href="http://www.w3.org/community/">W3C Community and Business Groups</a>. </p>
   <p></p>
  </div>
  <div data-fill-with="at-risk"></div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#intro"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li><a href="#definition"><span class="secno">2</span> <span class="content">Infrastructure definition</span></a>
    <li>
     <a href="#environment-settings-object-processing"><span class="secno">3</span> <span class="content">Environment settings object processing</span></a>
     <ol class="toc">
      <li><a href="#concept-client-hints-set"><span class="secno">3.1</span> <span class="content">Client hints set</span></a>
      <li><a href="#accept-ch-cache-definition"><span class="secno">3.2</span> <span class="content">Accept-CH cache</span></a>
      <li><a href="#initialize-ch-set"><span class="secno">3.3</span> <span class="content">Initialize Client Hints set</span></a>
      <li><a href="#accept-ch-state-algo"><span class="secno">3.4</span> <span class="content"><span>Accept-CH state</span> (<code>http-equiv="accept-ch"</code>)</span></a>
     </ol>
    <li>
     <a href="#html"><span class="secno">4</span> <span class="content">Integration with HTML</span></a>
     <ol class="toc">
      <li><a href="#document-init"><span class="secno">4.1</span> <span class="content">Document object initialization</span></a>
      <li><a href="#worker-init"><span class="secno">4.2</span> <span class="content">Worker initialization</span></a>
      <li><a href="#http-equiv-attributes"><span class="secno">4.3</span> <span class="content">http-equiv attributes</span></a>
      <li><a href="#pragma-directives"><span class="secno">4.4</span> <span class="content">Pragma directives</span></a>
      <li><a href="#extending-environment-settings-object"><span class="secno">4.5</span> <span class="content">Extending environment settings object</span></a>
     </ol>
    <li><a href="#request-processing"><span class="secno">5</span> <span class="content">Request processing</span></a>
    <li><a href="#fetch"><span class="secno">6</span> <span class="content">Integration with Fetch</span></a>
    <li>
     <a href="#registry"><span class="secno">7</span> <span class="content">Feature Registry</span></a>
     <ol class="toc">
      <li><a href="#client-hints-token-definition"><span class="secno">7.1</span> <span class="content">Client hints token</span></a>
      <li><a href="#policy-controlled-features"><span class="secno">7.2</span> <span class="content">Policy-controlled features</span></a>
      <li><a href="#low-entropy-table"><span class="secno">7.3</span> <span class="content">Low entropy hint table</span></a>
      <li><a href="#find-client-hint-value-section"><span class="secno">7.4</span> <span class="content">Find client hint value</span></a>
     </ol>
    <li><a href="#privacy"><span class="secno">8</span> <span class="content">Security and Privacy considerations</span></a>
    <li><a href="#terms"><span class="secno">9</span> <span class="content">Terms</span></a>
    <li><a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
    <li>
     <a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
     <ol class="toc">
      <li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
      <li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
     </ol>
    <li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>Client Hints is collection of HTTP and user-agent features that enables
privacy-preserving, proactive content negotiation with an explicit third-party
delegation mechanism:</p>
   <ul>
    <li data-md>
     <p>Proactive content negotiation at the HTTP layer enables servers to request
delivery of specific hints, in order to enable optimized and automated
selection of resources based on a user’s device, conditions and preferences,
and lets clients decide which hint requests they want to grant, with
per-hint and per-origin granularity.</p>
    <li data-md>
     <p>Integration of said mechanism with web concepts, defined in this document,
enables browsers to benefit from content adaptation, and have it play nicely with
current web restrictions (e.g. same-origin policy).</p>
    <li data-md>
     <p>The opt-in nature of the mechanism enables browsers to advertise requested
hint data (e.g. user agent and device characteristics) selectively to
secure-transport origins, instead of appending such data on every outgoing
request.</p>
    <li data-md>
     <p>Origin opt-in applies to same-origin assets only and delivery to third-party
origins is subject to explicit first party delegation via Feature Policy,
enabling tight control over which third party origins can access requested
hint data.</p>
   </ul>
   <p>The goal of Client Hints is to <strong>reduce passive fingerprinting</strong> on the web
while <strong>enabling scalable and privacy preserving content adaptation</strong> between
client and server, via a standardized set of content negotiation primitives at
the HTTP and user agent levels.</p>
   <h2 class="heading settled" data-level="2" id="definition"><span class="secno">2. </span><span class="content">Infrastructure definition</span><a class="self-link" href="#definition"></a></h2>
   <p>The specification of the Client Hints <strong>infrastructure</strong> is divided between the
following specifications and proposals:</p>
   <ul>
    <li data-md>
     <p>IETF <a data-link-type="biblio" href="#biblio-client-hints">[CLIENT-HINTS]</a></p>
     <ul>
      <li data-md>
       <p>Provides the motivation for Client Hints.</p>
      <li data-md>
       <p>Defines the fundamental Client Hints infrastructure:</p>
       <ul>
        <li data-md>
         <p>The <code>Accept-CH</code> response header, which servers may use to advertise
support for certain Client Hints.</p>
       </ul>
      <li data-md>
       <p>Provides both general guidelines, and formal requirements, about Client
 Hints’ impact on caching, security, and privacy.</p>
      <li data-md>
       <p>Does <em>not</em> define any actual, particular hints – or say anything about how
 Client Hints works in web contexts.</p>
     </ul>
    <li data-md>
     <p>Client Hints infrastructure - this document</p>
     <ul>
      <li data-md>
       <p>Defines how web clients should process the <code>Accept-CH</code> headers sent by servers.</p>
      <li data-md>
       <p>Defines the environment settings object state related to <code>Accept-CH</code>,
 which stores information about which servers should get which hints.</p>
      <li data-md>
       <p>Defines how, and when, web clients should actually go about sending hints,
 based on the state of their environment settings object.</p>
       <ul>
        <li data-md>
         <p>More specifically, it integrates the HTML web concepts with Fetch’s
algorithms to make sure that opted-in hints are added to requests for
same-origin or delegated-to cross-origin requests. It also makes sure
hints are removed from not delegated-to cross-origin requests after
redirections.</p>
       </ul>
      <li data-md>
       <p>Integrates those concepts with the <a data-link-type="biblio" href="#biblio-html">[HTML]</a> and <a data-link-type="biblio" href="#biblio-fetch">[FETCH]</a> specifications,
  by patching various concepts there.</p>
     </ul>
    <li data-md>
     <p>W3C Feature Policy specification (<a href="https://w3c.github.io/webappsec-feature-policy/#should-request-be-allowed-to-use-feature">relevant section</a>)</p>
     <ul>
      <li data-md>
       <p>In order to perform third party Client Hint delegation, Feature Policy has
 been extended to control features within fetch requests (rather than just Documents).</p>
     </ul>
   </ul>
   <h2 class="heading settled" data-level="3" id="environment-settings-object-processing"><span class="secno">3. </span><span class="content">Environment settings object processing</span><a class="self-link" href="#environment-settings-object-processing"></a></h2>
   <h3 class="heading settled" data-level="3.1" id="concept-client-hints-set"><span class="secno">3.1. </span><span class="content">Client hints set</span><a class="self-link" href="#concept-client-hints-set"></a></h3>
   <p>A <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="client-hints-set">client hints set</dfn> is a <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#ordered-set" id="ref-for-ordered-set">set</a> of <a data-link-type="dfn" href="#client-hints-token" id="ref-for-client-hints-token">client hints token</a>s. </p>
   <h3 class="heading settled" data-level="3.2" id="accept-ch-cache-definition"><span class="secno">3.2. </span><span class="content">Accept-CH cache</span><a class="self-link" href="#accept-ch-cache-definition"></a></h3>
   <p>An <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="accept-ch-cache">Accept-CH cache</dfn> is owned by the user agent and is an <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#ordered-map" id="ref-for-ordered-map">ordered map</a>, <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#map-key" id="ref-for-map-key">keyed</a> on <dfn class="dfn-paneled" data-dfn-for="accept-ch-cache" data-dfn-type="dfn" data-noexport id="accept-ch-cache-origin">origin</dfn> (an <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/origin.html#concept-origin" id="ref-for-concept-origin">origin</a>), 
  with a value of <dfn class="dfn-paneled" data-dfn-for="accept-ch-cache" data-dfn-type="dfn" data-noexport id="accept-ch-cache-client-hints-set">client hints set</dfn> (a <a data-link-type="dfn" href="#client-hints-set" id="ref-for-client-hints-set">client hints set</a>).</p>
   <p>The Accept-CH cache can effectively act as an alternative cookie store,
since sites can use each of the hints as a bit set on the client, and that information will be
communicated to them on every request. As such, a user agent MUST evict that
cache whenever the user clears their cookies or when session cookies expire.</p>
   <p>To <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="add-a-new-accept-ch-cache-entry">add a new Accept-CH cache entry</dfn> to the <a data-link-type="dfn" href="#accept-ch-cache" id="ref-for-accept-ch-cache">Accept-CH cache</a>,
given an <a data-link-type="dfn" href="#accept-ch-cache-origin" id="ref-for-accept-ch-cache-origin">origin</a> <var>origin</var> and a <a data-link-type="dfn" href="#client-hints-set" id="ref-for-client-hints-set①">client hints set</a> <var>client hints set</var>, <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#map-set" id="ref-for-map-set">set</a> <a data-link-type="dfn" href="#accept-ch-cache" id="ref-for-accept-ch-cache①">Accept-CH cache</a>[<var>origin</var>] to <var>client hints set</var>.</p>
   <p>To <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="retrieve-the-client-hints-set">retrieve the client hints set</dfn> given an <var>origin</var>:</p>
   <ol>
    <li data-md>
     <p>Let <var>clientHintsSet</var> be an empty <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#ordered-set" id="ref-for-ordered-set①">ordered set</a>.</p>
    <li data-md>
     <p>Let <var>originMatchingEntries</var> be the entries in the <a data-link-type="dfn" href="#accept-ch-cache" id="ref-for-accept-ch-cache②">Accept-CH cache</a> whose <a data-link-type="dfn" href="#accept-ch-cache-origin" id="ref-for-accept-ch-cache-origin①">origin</a> is <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/origin.html#same-origin" id="ref-for-same-origin">same origin</a> with <var>origin</var>.</p>
    <li data-md>
     <p>For each entry in <var>originMatchingEntries</var>, for each token in its <a data-link-type="dfn" href="#accept-ch-cache-client-hints-set" id="ref-for-accept-ch-cache-client-hints-set">client hints set</a>, <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#set-append" id="ref-for-set-append">append</a> the token to <var>clientHintsSet</var>.</p>
    <li data-md>
     <p>Return <var>clientHintsSet</var>.</p>
   </ol>
   <h3 class="heading settled" data-level="3.3" id="initialize-ch-set"><span class="secno">3.3. </span><span class="content">Initialize Client Hints set</span><a class="self-link" href="#initialize-ch-set"></a></h3>
    When asked to <dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export id="abstract-opdef-initialize-the-client-hints-set">initialize the Client Hints set</dfn> with <var>settingsObject</var> and <var>response</var> as inputs, run the following steps: 
   <ol>
    <li data-md>
     <p>Let <var>clientHintsSet</var> be the result of running <a data-link-type="dfn" href="#retrieve-the-client-hints-set" id="ref-for-retrieve-the-client-hints-set">retrieve the client hints set</a> with <var>settingsObject</var>’s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#concept-settings-object-origin" id="ref-for-concept-settings-object-origin">origin</a>.</p>
    <li data-md>
     <p>For each <var>hint</var> in <var>clientHintsSet</var>, <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#set-append" id="ref-for-set-append①">append</a> <var>hint</var> to <var>settingsObject</var>’s <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set">client hints set</a>.</p>
    <li data-md>
     <p>If the result of executing <a data-link-type="abstract-op" href="https://w3c.github.io/webappsec-secure-contexts/#settings-object" id="ref-for-settings-object">Is an environment settings object contextually secure?</a> on <var>settingsObject</var> is <code>"Not Secure"</code>, abort these steps.</p>
    <li data-md>
     <p>Let <var>browsingContext</var> be <var>settingsObject</var>’s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#responsible-browsing-context" id="ref-for-responsible-browsing-context">responsible browsing context</a>.</p>
    <li data-md>
     <p>If the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/browsers.html#top-level-browsing-context" id="ref-for-top-level-browsing-context">top-level browsing context</a> does not equal <var>browsingContext</var>, abort these steps.</p>
    <li data-md>
     <p>If <var>response</var>’s <code>Accept-CH</code> header is present, parse the header field value according to the <code>Accept-CH</code> header parsing rules, as a <a data-link-type="dfn" href="#field-name" id="ref-for-field-name">field-name</a>. Add each parsed <a data-link-type="dfn" href="#client-hints-token" id="ref-for-client-hints-token①">client hints token</a> to <var>settingsObject</var>’s <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set①">client hints set</a>.</p>
    <li data-md>
     <p><a data-link-type="dfn" href="#add-a-new-accept-ch-cache-entry" id="ref-for-add-a-new-accept-ch-cache-entry">Add a new Accept-CH cache entry</a> with <var>response</var>’s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/origin.html#concept-origin" id="ref-for-concept-origin①">origin</a> and <var>settingsObject</var>’s <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set②">client hints set</a> as inputs.</p>
   </ol>
   <div class="note" role="note">
     Note, the above algorithm: 
    <ul>
     <li data-md>
      <p>Initializes client hints set on the environment settings object based on its origin.</p>
     <li data-md>
      <p>If we are in a secure context and the navigation is a top-level navigation,
it parses <code>Accept-CH</code> and adds the results to the environment setting object’s client hints set as well as the Accept-CH cache.</p>
    </ul>
   </div>
   <h3 class="heading settled" data-level="3.4" id="accept-ch-state-algo"><span class="secno">3.4. </span><span class="content"><dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="accept-ch-state">Accept-CH state</dfn> (<code>http-equiv="accept-ch"</code>)</span><a class="self-link" href="#accept-ch-state-algo"></a></h3>
   <p class="note" role="note"><span>Note:</span> This pragma appends <a data-link-type="dfn" href="#client-hints-token" id="ref-for-client-hints-token②">client hints token</a>s to the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object" id="ref-for-environment-settings-object">environment settings object</a>'s <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set③">client hints set</a>.</p>
   <ol>
    <li data-md>
     <p>If the <code><a data-link-type="element" href="https://html.spec.whatwg.org/multipage/semantics.html#meta" id="ref-for-meta">meta</a></code> element is not a child of a <code><a data-link-type="element" href="https://html.spec.whatwg.org/multipage/semantics.html#the-head-element" id="ref-for-the-head-element">head</a></code> element, then return.</p>
    <li data-md>
     <p>If the <code><a data-link-type="element" href="https://html.spec.whatwg.org/multipage/semantics.html#meta" id="ref-for-meta①">meta</a></code> element has no <code><a data-link-type="element-sub" href="https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-content" id="ref-for-attr-meta-content">content</a></code> attribute, or if that attribute’s value is the empty string, then return.</p>
    <li data-md>
     <p>Let <var>settingsObject</var> be the <code><a data-link-type="element" href="https://html.spec.whatwg.org/multipage/semantics.html#meta" id="ref-for-meta②">meta</a></code> element’s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#relevant-settings-object" id="ref-for-relevant-settings-object">relevant settings object</a>.</p>
    <li data-md>
     <p>If the result of executing <a data-link-type="abstract-op" href="https://w3c.github.io/webappsec-secure-contexts/#settings-object" id="ref-for-settings-object①">Is an environment settings object contextually secure?</a> on <var>settingsObject</var> is <code>"Not Secure"</code>, then return.</p>
    <li data-md>
     <p>Let <var>browsingContext</var> be <var>settingsObject</var>’s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#responsible-browsing-context" id="ref-for-responsible-browsing-context①">responsible browsing context</a>.</p>
    <li data-md>
     <p>If the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/browsers.html#top-level-browsing-context" id="ref-for-top-level-browsing-context①">top-level browsing context</a> does not equal <var>browsingContext</var>, abort these steps.</p>
    <li data-md>
     <p>Let <var>acceptCHValue</var> be the <code><a data-link-type="element" href="https://html.spec.whatwg.org/multipage/semantics.html#meta" id="ref-for-meta③">meta</a></code> element’s <code><a data-link-type="element-sub" href="https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-content" id="ref-for-attr-meta-content①">content</a></code> attribute’s value.</p>
    <li data-md>
     <p>Parse <var>acceptCHValue</var> according to the <a data-link-type="dfn" href="#accept-ch" id="ref-for-accept-ch">Accept-CH</a> header parsing rules, as a <a data-link-type="dfn" href="#field-name" id="ref-for-field-name①">field-name</a>. <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#set-append" id="ref-for-set-append②">Append</a> each parsed <a data-link-type="dfn" href="#client-hints-token" id="ref-for-client-hints-token③">client hints token</a> to <var>settingsObject</var>’s <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set④">client hints set</a>.</p>
    <li data-md>
     <p><a data-link-type="dfn" href="#add-a-new-accept-ch-cache-entry" id="ref-for-add-a-new-accept-ch-cache-entry①">Add a new Accept-CH cache entry</a> with <var>settingsObject</var>’s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/origin.html#concept-origin" id="ref-for-concept-origin②">origin</a>, and <var>settingsObject</var>’s <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set⑤">client hints set</a>.</p>
   </ol>
   <h2 class="heading settled" data-level="4" id="html"><span class="secno">4. </span><span class="content">Integration with HTML</span><a class="self-link" href="#html"></a></h2>
   <p>This specification integrates with the <a data-link-type="biblio" href="#biblio-html">[HTML]</a> specification by patching the algorithms below:</p>
   <h3 class="heading settled" data-level="4.1" id="document-init"><span class="secno">4.1. </span><span class="content">Document object initialization</span><a class="self-link" href="#document-init"></a></h3>
   <p>At <a href="https://html.spec.whatwg.org/multipage/browsing-the-web.html#initialise-the-document-object">Create and initialize a Document object</a>,
after step 11, starting with "Initialize a Document’s CSP list", 
call <a data-link-type="abstract-op" href="#abstract-opdef-initialize-the-client-hints-set" id="ref-for-abstract-opdef-initialize-the-client-hints-set">initialize the Client Hints set</a> with <var>document</var>’s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#relevant-settings-object" id="ref-for-relevant-settings-object①">relevant settings object</a> and <var>response</var> as inputs.</p>
   <h3 class="heading settled" data-level="4.2" id="worker-init"><span class="secno">4.2. </span><span class="content">Worker initialization</span><a class="self-link" href="#worker-init"></a></h3>
    At <a href="https://html.spec.whatwg.org/multipage/workers.html#set-up-a-worker-environment-settings-object">set up a worker environment settings object</a>,
after step 6, add the following step: 
   <ol>
    <li data-md>
     <p>Set <var>settingsObject</var>’s <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set⑥">client hints set</a> to be a <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-clone" id="ref-for-list-clone">clone</a> of <var>outside settings</var>’ <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set⑦">client hints set</a>.</p>
   </ol>
   <h3 class="heading settled" data-level="4.3" id="http-equiv-attributes"><span class="secno">4.3. </span><span class="content">http-equiv attributes</span><a class="self-link" href="#http-equiv-attributes"></a></h3>
   <div class="non-normative">
     <em>This section is non-normative.</em> 
    <p>In the table on <a href="https://html.spec.whatwg.org/multipage/#attributes-3">attributes</a>, add "accept-ch" in the line which attribute is "http-equiv".</p>
   </div>
   <h3 class="heading settled" data-level="4.4" id="pragma-directives"><span class="secno">4.4. </span><span class="content">Pragma directives</span><a class="self-link" href="#pragma-directives"></a></h3>
    For the table in <a href="https://html.spec.whatwg.org/multipage/#pragma-directives">pragma directives</a>,
add a line with a "State" value of <a data-link-type="dfn" href="#accept-ch-state" id="ref-for-accept-ch-state">Accept-CH</a> and a "Keyword" value of <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="accept-ch">accept-ch</dfn>. 
   <h3 class="heading settled" data-level="4.5" id="extending-environment-settings-object"><span class="secno">4.5. </span><span class="content">Extending environment settings object</span><a class="self-link" href="#extending-environment-settings-object"></a></h3>
   <p>An <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object" id="ref-for-environment-settings-object①">environment settings object</a> has a <dfn class="dfn-paneled" data-dfn-for="environment settings object" data-dfn-type="dfn" data-noexport id="environment-settings-object-client-hints-set">client hints set</dfn>: a <a data-link-type="dfn" href="#client-hints-set" id="ref-for-client-hints-set②">client hints set</a>, initially the empty set, used for <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-fetch" id="ref-for-concept-fetch">fetches</a> performed using the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object" id="ref-for-environment-settings-object②">environment settings object</a> as a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-request" id="ref-for-concept-request-request">request</a> <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-client" id="ref-for-concept-request-client">client</a>.</p>
   <h2 class="heading settled" data-level="5" id="request-processing"><span class="secno">5. </span><span class="content">Request processing</span><a class="self-link" href="#request-processing"></a></h2>
   <p>When asked to <dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export id="abstract-opdef-append-client-hints-to-request">append client hints to request</dfn> with <var>request</var> as input, run the following steps:</p>
   <p>If <var>request</var> is a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#navigation-request" id="ref-for-navigation-request">navigation request</a>, a user agent should, for each <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-header" id="ref-for-concept-header">header</a> <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-header-name" id="ref-for-concept-header-name">name</a> (<var>hintName</var>) in the registry’s <a data-link-type="dfn" href="#low-entropy-hint-table" id="ref-for-low-entropy-hint-table">low entropy hint table</a>,
if <var>request</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list">header list</a> <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#header-list-contains" id="ref-for-header-list-contains">does not contain</a> <var>hintName</var>, then <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-header-list-append" id="ref-for-concept-header-list-append">append</a> <var>hintName</var>/the corresponding value to <var>request</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list①">header list</a>. </p>
   <p class="issue" id="issue-f40790e6"><a class="self-link" href="#issue-f40790e6"></a> Define registry’s low entropy table</p>
   <p>If <var>request</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-client" id="ref-for-concept-request-client①">client</a> is not null,
  then <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-iterate" id="ref-for-list-iterate">for each</a> <a data-link-type="dfn" href="#client-hints-token" id="ref-for-client-hints-token④">client hints token</a> <var>hintName</var> of <var>request</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-client" id="ref-for-concept-request-client②">client</a>’s <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set⑧">client hints set</a>: </p>
   <ol>
    <li>
     <p>Let <var>value</var> be the return value of running <a data-link-type="dfn" href="#find-client-hint-value" id="ref-for-find-client-hint-value">find client hint value</a>, given <var>hintName</var> as input. </p>
    <li>
     <p>If <var>request</var> is a <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#subresource-request" id="ref-for-subresource-request">subresource request</a> and the result of running <a href="https://w3c.github.io/webappsec-feature-policy/#algo-should-request-be-allowed-to-use-feature">Should request be allowed to use feature?</a>,
   given <var>request</var> and <var>hintName</var>’s <a href="http://httpwg.org/http-extensions/client-hints.html#opt-in-via-feature-policy">associated
   policy-controlled feature</a>, returns <code>false</code>, then skip the next steps and
   continue to the next <var>hintName</var>. <a data-link-type="biblio" href="#biblio-feature-policy">[FEATURE-POLICY]</a> <a data-link-type="biblio" href="#biblio-client-hints">[CLIENT-HINTS]</a> </p>
    <li>
     <p>Set <var>hintName</var> to "Sec-" concatenated with <var>hintName</var>. </p>
     <div class="issue" id="issue-48a63eb6"><a class="self-link" href="#issue-48a63eb6"></a>We need to figure out if we really want a <code>Sec-</code> prefix, and if so also exempt it from CORS.</div>
    <li>
     <p>If <var>request</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list②">header list</a> <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#header-list-contains" id="ref-for-header-list-contains①">does not
  contain</a> <var>hintName</var>, a user agent should <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-header-list-append" id="ref-for-concept-header-list-append①">append</a> <var>hintName</var>/<var>value</var> to <var>request</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list③">header list</a>. </p>
   </ol>
   <p>When asked to <dfn class="dfn-paneled" data-dfn-type="abstract-op" data-export id="abstract-opdef-remove-client-hints-from-redirect-if-needed">remove client hints from redirect if needed</dfn> with <var>request</var> as input, run the following steps:</p>
   <ol>
    <li>If <var>request</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-client" id="ref-for-concept-request-client③">client</a> is null, then abort these steps. 
    <li>Let <var>clientHintsSet</var> be <var>request</var>’s <var>client</var>’s <a data-link-type="dfn" href="#environment-settings-object-client-hints-set" id="ref-for-environment-settings-object-client-hints-set⑨">client hints set</a>. 
    <li>
     <p><a data-link-type="dfn" href="https://infra.spec.whatwg.org/#list-iterate" id="ref-for-list-iterate①">For each</a> <var>hintName</var> of <var>clientHintsSet</var>: </p>
     <ol>
      <li>
       <p>Set <var>hintName</var> to "Sec-" concatenated with <var>hintName</var>. </p>
      <li>
       <p>If <var>request</var>’s <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list④">header list</a> <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#header-list-contains" id="ref-for-header-list-contains②">contains</a> <var>hintName</var> and if the result of running <a href="https://wicg.github.io/feature-policy/#should-request-be-allowed-to-use-feature">Should
  request be allowed to use feature?</a>, given <var>request</var> and <var>hintName</var>’s <a href="http://httpwg.org/http-extensions/client-hints.html#opt-in-via-feature-policy">associated
  policy-controlled feature</a>, returns <code>false</code>, then remove <var>hintName</var> from <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-request-header-list" id="ref-for-concept-request-header-list⑤">header list</a>. <a data-link-type="biblio" href="#biblio-feature-policy">[FEATURE-POLICY]</a> <a data-link-type="biblio" href="#biblio-client-hints">[CLIENT-HINTS]</a> </p>
     </ol>
   </ol>
   <h2 class="heading settled" data-level="6" id="fetch"><span class="secno">6. </span><span class="content">Integration with Fetch</span><a class="self-link" href="#fetch"></a></h2>
   <p>This specification integrates with the <a data-link-type="biblio" href="#biblio-fetch">[FETCH]</a> specification by patching the algorithms below:</p>
   <p>In <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-fetch" id="ref-for-concept-fetch①">Fetching</a>, after step 1.6, run <a data-link-type="abstract-op" href="#abstract-opdef-append-client-hints-to-request" id="ref-for-abstract-opdef-append-client-hints-to-request">append client hints to request</a> with <var>request</var> as input.</p>
   <p>In <a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-http-redirect-fetch" id="ref-for-concept-http-redirect-fetch">HTTP-redirect fetch</a>, after step 7, run <a data-link-type="abstract-op" href="#abstract-opdef-remove-client-hints-from-redirect-if-needed" id="ref-for-abstract-opdef-remove-client-hints-from-redirect-if-needed">remove client hints from redirect if needed</a> with <var>request</var> as input.</p>
   <h2 class="heading settled" data-level="7" id="registry"><span class="secno">7. </span><span class="content">Feature Registry</span><a class="self-link" href="#registry"></a></h2>
   <p class="note" role="note"><span>Note:</span> This section contains feature-specific definitions.
  New features that rely on the Client Hints infrastructure need to add their respective definitions to this registry.
  User Agents can implement some of those features without implementing others.</p>
   <h3 class="heading settled" data-level="7.1" id="client-hints-token-definition"><span class="secno">7.1. </span><span class="content">Client hints token</span><a class="self-link" href="#client-hints-token-definition"></a></h3>
   <p>A <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="client-hints-token">client hints token</dfn> is a <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#byte-lowercase" id="ref-for-byte-lowercase">byte-lowercase</a> representation of one of <code>Save-Data</code>, <code>DPR</code>, <code>Width</code>, <code>Viewport-Width</code>, <code>Device-Memory</code>, <code>RTT</code>, <code>Downlink</code>, <code>ECT</code>, <code>Arch</code>, <code>Model</code>, <code>Platform</code>, <code>UA</code> or <code>Mobile</code>.</p>
   <h3 class="heading settled" data-level="7.2" id="policy-controlled-features"><span class="secno">7.2. </span><span class="content">Policy-controlled features</span><a class="self-link" href="#policy-controlled-features"></a></h3>
   <p>This document defines the following <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#policy-controlled-feature" id="ref-for-policy-controlled-feature">policy-controlled features</a>:</p>
   <ul>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-save-data">ch-save-data<a class="self-link" href="#ch-save-data"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist">default allowlist</a> of <code>'*'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-dpr">ch-dpr<a class="self-link" href="#ch-dpr"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist①">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-width">ch-width<a class="self-link" href="#ch-width"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist②">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-viewport-width">ch-viewport-width<a class="self-link" href="#ch-viewport-width"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist③">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-device-memory">ch-device-memory<a class="self-link" href="#ch-device-memory"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist④">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-rtt">ch-rtt<a class="self-link" href="#ch-rtt"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist⑤">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-downlink">ch-downlink<a class="self-link" href="#ch-downlink"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist⑥">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-ect">ch-ect<a class="self-link" href="#ch-ect"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist⑦">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-arch">ch-arch<a class="self-link" href="#ch-arch"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist⑧">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-model">ch-model<a class="self-link" href="#ch-model"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist⑨">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-platform">ch-platform<a class="self-link" href="#ch-platform"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist①⓪">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-ua">ch-ua<a class="self-link" href="#ch-ua"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist①①">default allowlist</a> of <code>'self'</code></p>
    <li data-md>
     <p><code><dfn data-dfn-type="dfn" data-export id="ch-mobile">ch-mobile<a class="self-link" href="#ch-mobile"></a></dfn></code> which has a <a data-link-type="dfn" href="https://wicg.github.io/feature-policy/#default-allowlist" id="ref-for-default-allowlist①②">default allowlist</a> of <code>'self'</code></p>
   </ul>
   <p class="issue" id="issue-ef477b6d"><a class="self-link" href="#issue-ef477b6d"></a> Should Save-data really have an allowlist of <a class="property" data-link-type="propdesc">*</a>? If so, is it because it’s somehow "low entropy" and should we tie low-entropy-ness to allowlists, generally?</p>
   <h3 class="heading settled" data-level="7.3" id="low-entropy-table"><span class="secno">7.3. </span><span class="content">Low entropy hint table</span><a class="self-link" href="#low-entropy-table"></a></h3>
    The <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="low-entropy-hint-table">low entropy hint table</dfn> below defines hints that are only exposing low amounts of entropy. 
   <table>
    <thead>
     <tr>
      <th><a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-header-name" id="ref-for-concept-header-name①">Name</a> 
      <th><a data-link-type="dfn" href="https://fetch.spec.whatwg.org/#concept-header-value" id="ref-for-concept-header-value">Value</a> 
    <tbody>
     <tr>
      <td><code>Save-Data</code> 
      <td>a suitable <a href="https://wicg.github.io/savedata/#save-data-request-header-field">Save-Data value</a> 
   </table>
   <h3 class="heading settled" data-level="7.4" id="find-client-hint-value-section"><span class="secno">7.4. </span><span class="content">Find client hint value</span><a class="self-link" href="#find-client-hint-value-section"></a></h3>
   <p>When asked to <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="find-client-hint-value">find client hint value</dfn>, given <var>hint</var> as input, switch on <var>hint</var> and return the result:</p>
   <dl class="switch">
    <dt><code>Save-Data</code> 
    <dd>a suitable <a href="https://wicg.github.io/savedata/#save-data-request-header-field">Save-Data value</a> 
    <dt><code>DPR</code> 
    <dd>a suitable <a href>DPR value</a> 
    <dt><code>Viewport-Width</code> 
    <dd>a suitable <a href>Viewport-Width value</a> 
    <dt><code>Width</code> 
    <dd>a suitable <a href>Width value</a> 
    <dt><code>Device-Memory</code> 
    <dd>a suitable <a href="https://w3c.github.io/device-memory/#sec-device-memory-client-hint-header">Device-Memory value</a> 
    <dt><code>RTT</code> 
    <dd>a suitable <a href="https://wicg.github.io/netinfo/#rtt-request-header-field">RTT value</a> 
    <dt><code>Downlink</code> 
    <dd>a suitable <a href="https://wicg.github.io/netinfo/#downlink-request-header-field">Downlink value</a> 
    <dt><code>ECT</code> 
    <dd>a suitable <a href="https://wicg.github.io/netinfo/#ect-request-header-field">ECT value</a> 
    <dt><code>Arch</code> 
    <dd>a suitable <a href="https://wicg.github.io/ua-client-hints/#sec-ch-arch">Arch value</a> 
    <dt><code>Model</code> 
    <dd>a suitable <a href="https://wicg.github.io/ua-client-hints/#sec-ch-model">Model value</a> 
    <dt><code>Platform</code> 
    <dd>a suitable <a href="https://wicg.github.io/ua-client-hints/#sec-ch-platform">Platform value</a> 
    <dt><code>UA</code> 
    <dd>a suitable <a href="https://wicg.github.io/ua-client-hints/#sec-ch-ua">UA value</a> 
    <dt><code>Mobile</code> 
    <dd>a suitable <a href="https://wicg.github.io/ua-client-hints/#sec-ch-mobile">Mobile value</a> 
   </dl>
   <p class="issue" id="issue-707decdc"><a class="self-link" href="#issue-707decdc"></a> Links for image features are broken, need to actually define that and link to them.</p>
   <h2 class="heading settled" data-level="8" id="privacy"><span class="secno">8. </span><span class="content">Security and Privacy considerations</span><a class="self-link" href="#privacy"></a></h2>
    See <a data-link-type="biblio" href="#biblio-client-hints">[CLIENT-HINTS]</a>. 
   <h2 class="heading settled" data-level="9" id="terms"><span class="secno">9. </span><span class="content">Terms</span><a class="self-link" href="#terms"></a></h2>
   <p>The following terms are defined in the HTTP specifications: <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport href="https://tools.ietf.org/html/rfc7230#section-3.2" id="field-name">field-name</dfn></p>
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  <div data-fill-with="conformance">
   <h2 class="no-ref no-num heading settled" id="conformance"><span class="content"> Conformance</span><a class="self-link" href="#conformance"></a></h2>
   <p> Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology.
            The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL”
            in the normative parts of this document
            are to be interpreted as described in RFC 2119.
            However, for readability,
            these words do not appear in all uppercase letters in this specification. </p>
   <p> All of the text of this specification is normative
            except sections explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-rfc2119">[RFC2119]</a> </p>
   <p> Examples in this specification are introduced with the words “for example”
            or are set apart from the normative text with <code>class="example"</code>, like this: </p>
   <div class="example" id="example-example"><a class="self-link" href="#example-example"></a> This is an example of an informative example. </div>
   <p> Informative notes begin with the word “Note”
            and are set apart from the normative text with <code>class="note"</code>, like this: </p>
   <p class="note" role="note"> Note, this is an informative note. </p>
  </div>
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  <h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
  <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
  <ul class="index">
   <li><a href="#accept-ch">accept-ch</a><span>, in §4.4</span>
   <li><a href="#accept-ch-cache">Accept-CH cache</a><span>, in §3.2</span>
   <li><a href="#accept-ch-state">Accept-CH state</a><span>, in §3.4</span>
   <li><a href="#add-a-new-accept-ch-cache-entry">add a new Accept-CH cache entry</a><span>, in §3.2</span>
   <li><a href="#abstract-opdef-append-client-hints-to-request">append client hints to request</a><span>, in §5</span>
   <li><a href="#ch-arch">ch-arch</a><span>, in §7.2</span>
   <li><a href="#ch-device-memory">ch-device-memory</a><span>, in §7.2</span>
   <li><a href="#ch-downlink">ch-downlink</a><span>, in §7.2</span>
   <li><a href="#ch-dpr">ch-dpr</a><span>, in §7.2</span>
   <li><a href="#ch-ect">ch-ect</a><span>, in §7.2</span>
   <li><a href="#ch-mobile">ch-mobile</a><span>, in §7.2</span>
   <li><a href="#ch-model">ch-model</a><span>, in §7.2</span>
   <li><a href="#ch-platform">ch-platform</a><span>, in §7.2</span>
   <li><a href="#ch-rtt">ch-rtt</a><span>, in §7.2</span>
   <li><a href="#ch-save-data">ch-save-data</a><span>, in §7.2</span>
   <li><a href="#ch-ua">ch-ua</a><span>, in §7.2</span>
   <li><a href="#ch-viewport-width">ch-viewport-width</a><span>, in §7.2</span>
   <li><a href="#ch-width">ch-width</a><span>, in §7.2</span>
   <li>
    client hints set
    <ul>
     <li><a href="#client-hints-set">definition of</a><span>, in §3.1</span>
     <li><a href="#accept-ch-cache-client-hints-set">dfn for accept-ch-cache</a><span>, in §3.2</span>
     <li><a href="#environment-settings-object-client-hints-set">dfn for environment settings object</a><span>, in §4.5</span>
    </ul>
   <li><a href="#client-hints-token">client hints token</a><span>, in §7.1</span>
   <li><a href="#field-name">field-name</a><span>, in §9</span>
   <li><a href="#find-client-hint-value">find client hint value</a><span>, in §7.4</span>
   <li><a href="#abstract-opdef-initialize-the-client-hints-set">initialize the Client Hints set</a><span>, in §3.3</span>
   <li><a href="#low-entropy-hint-table">low entropy hint table</a><span>, in §7.3</span>
   <li><a href="#accept-ch-cache-origin">origin</a><span>, in §3.2</span>
   <li><a href="#abstract-opdef-remove-client-hints-from-redirect-if-needed">remove client hints from redirect if needed</a><span>, in §5</span>
   <li><a href="#retrieve-the-client-hints-set">retrieve the client hints set</a><span>, in §3.2</span>
  </ul>
  <aside class="dfn-panel" data-for="term-for-default-allowlist">
   <a href="https://wicg.github.io/feature-policy/#default-allowlist">https://wicg.github.io/feature-policy/#default-allowlist</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-default-allowlist">7.2. Policy-controlled features</a> <a href="#ref-for-default-allowlist①">(2)</a> <a href="#ref-for-default-allowlist②">(3)</a> <a href="#ref-for-default-allowlist③">(4)</a> <a href="#ref-for-default-allowlist④">(5)</a> <a href="#ref-for-default-allowlist⑤">(6)</a> <a href="#ref-for-default-allowlist⑥">(7)</a> <a href="#ref-for-default-allowlist⑦">(8)</a> <a href="#ref-for-default-allowlist⑧">(9)</a> <a href="#ref-for-default-allowlist⑨">(10)</a> <a href="#ref-for-default-allowlist①⓪">(11)</a> <a href="#ref-for-default-allowlist①①">(12)</a> <a href="#ref-for-default-allowlist①②">(13)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-policy-controlled-feature">
   <a href="https://wicg.github.io/feature-policy/#policy-controlled-feature">https://wicg.github.io/feature-policy/#policy-controlled-feature</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-policy-controlled-feature">7.2. Policy-controlled features</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-header-list-append">
   <a href="https://fetch.spec.whatwg.org/#concept-header-list-append">https://fetch.spec.whatwg.org/#concept-header-list-append</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-header-list-append">5. Request processing</a> <a href="#ref-for-concept-header-list-append①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-request-client">
   <a href="https://fetch.spec.whatwg.org/#concept-request-client">https://fetch.spec.whatwg.org/#concept-request-client</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-request-client">4.5. Extending environment settings object</a>
    <li><a href="#ref-for-concept-request-client①">5. Request processing</a> <a href="#ref-for-concept-request-client②">(2)</a> <a href="#ref-for-concept-request-client③">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-header-list-contains">
   <a href="https://fetch.spec.whatwg.org/#header-list-contains">https://fetch.spec.whatwg.org/#header-list-contains</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-header-list-contains">5. Request processing</a> <a href="#ref-for-header-list-contains①">(2)</a> <a href="#ref-for-header-list-contains②">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-header-list-contains">
   <a href="https://fetch.spec.whatwg.org/#header-list-contains">https://fetch.spec.whatwg.org/#header-list-contains</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-header-list-contains">5. Request processing</a> <a href="#ref-for-header-list-contains①">(2)</a> <a href="#ref-for-header-list-contains②">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-fetch">
   <a href="https://fetch.spec.whatwg.org/#concept-fetch">https://fetch.spec.whatwg.org/#concept-fetch</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-fetch">4.5. Extending environment settings object</a>
    <li><a href="#ref-for-concept-fetch①">6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-header">
   <a href="https://fetch.spec.whatwg.org/#concept-header">https://fetch.spec.whatwg.org/#concept-header</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-header">5. Request processing</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-request-header-list">
   <a href="https://fetch.spec.whatwg.org/#concept-request-header-list">https://fetch.spec.whatwg.org/#concept-request-header-list</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-request-header-list">5. Request processing</a> <a href="#ref-for-concept-request-header-list①">(2)</a> <a href="#ref-for-concept-request-header-list②">(3)</a> <a href="#ref-for-concept-request-header-list③">(4)</a> <a href="#ref-for-concept-request-header-list④">(5)</a> <a href="#ref-for-concept-request-header-list⑤">(6)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-http-redirect-fetch">
   <a href="https://fetch.spec.whatwg.org/#concept-http-redirect-fetch">https://fetch.spec.whatwg.org/#concept-http-redirect-fetch</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-http-redirect-fetch">6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-header-name">
   <a href="https://fetch.spec.whatwg.org/#concept-header-name">https://fetch.spec.whatwg.org/#concept-header-name</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-header-name">5. Request processing</a>
    <li><a href="#ref-for-concept-header-name①">7.3. Low entropy hint table</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-navigation-request">
   <a href="https://fetch.spec.whatwg.org/#navigation-request">https://fetch.spec.whatwg.org/#navigation-request</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-navigation-request">5. Request processing</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-request-request">
   <a href="https://fetch.spec.whatwg.org/#concept-request-request">https://fetch.spec.whatwg.org/#concept-request-request</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-request-request">4.5. Extending environment settings object</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-subresource-request">
   <a href="https://fetch.spec.whatwg.org/#subresource-request">https://fetch.spec.whatwg.org/#subresource-request</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-subresource-request">5. Request processing</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-header-value">
   <a href="https://fetch.spec.whatwg.org/#concept-header-value">https://fetch.spec.whatwg.org/#concept-header-value</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-header-value">7.3. Low entropy hint table</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-attr-meta-content">
   <a href="https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-content">https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-content</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-attr-meta-content">3.4. Accept-CH state (http-equiv="accept-ch")</a> <a href="#ref-for-attr-meta-content①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-environment-settings-object">
   <a href="https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object">https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-environment-settings-object">3.4. Accept-CH state (http-equiv="accept-ch")</a>
    <li><a href="#ref-for-environment-settings-object①">4.5. Extending environment settings object</a> <a href="#ref-for-environment-settings-object②">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-the-head-element">
   <a href="https://html.spec.whatwg.org/multipage/semantics.html#the-head-element">https://html.spec.whatwg.org/multipage/semantics.html#the-head-element</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-the-head-element">3.4. Accept-CH state (http-equiv="accept-ch")</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-meta">
   <a href="https://html.spec.whatwg.org/multipage/semantics.html#meta">https://html.spec.whatwg.org/multipage/semantics.html#meta</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-meta">3.4. Accept-CH state (http-equiv="accept-ch")</a> <a href="#ref-for-meta①">(2)</a> <a href="#ref-for-meta②">(3)</a> <a href="#ref-for-meta③">(4)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-concept-settings-object-origin">
   <a href="https://html.spec.whatwg.org/multipage/webappapis.html#concept-settings-object-origin">https://html.spec.whatwg.org/multipage/webappapis.html#concept-settings-object-origin</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-concept-settings-object-origin">3.3. Initialize Client Hints set</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-relevant-settings-object">
   <a href="https://html.spec.whatwg.org/multipage/webappapis.html#relevant-settings-object">https://html.spec.whatwg.org/multipage/webappapis.html#relevant-settings-object</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-relevant-settings-object">3.4. Accept-CH state (http-equiv="accept-ch")</a>
    <li><a href="#ref-for-relevant-settings-object①">4.1. Document object initialization</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-responsible-browsing-context">
   <a href="https://html.spec.whatwg.org/multipage/webappapis.html#responsible-browsing-context">https://html.spec.whatwg.org/multipage/webappapis.html#responsible-browsing-context</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-responsible-browsing-context">3.3. Initialize Client Hints set</a>
    <li><a href="#ref-for-responsible-browsing-context①">3.4. Accept-CH state (http-equiv="accept-ch")</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-same-origin">
   <a href="https://html.spec.whatwg.org/multipage/origin.html#same-origin">https://html.spec.whatwg.org/multipage/origin.html#same-origin</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-same-origin">3.2. Accept-CH cache</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-top-level-browsing-context">
   <a href="https://html.spec.whatwg.org/multipage/browsers.html#top-level-browsing-context">https://html.spec.whatwg.org/multipage/browsers.html#top-level-browsing-context</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-top-level-browsing-context">3.3. Initialize Client Hints set</a>
    <li><a href="#ref-for-top-level-browsing-context①">3.4. Accept-CH state (http-equiv="accept-ch")</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-set-append">
   <a href="https://infra.spec.whatwg.org/#set-append">https://infra.spec.whatwg.org/#set-append</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-set-append">3.2. Accept-CH cache</a>
    <li><a href="#ref-for-set-append①">3.3. Initialize Client Hints set</a>
    <li><a href="#ref-for-set-append②">3.4. Accept-CH state (http-equiv="accept-ch")</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-byte-lowercase">
   <a href="https://infra.spec.whatwg.org/#byte-lowercase">https://infra.spec.whatwg.org/#byte-lowercase</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-byte-lowercase">7.1. Client hints token</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-list-clone">
   <a href="https://infra.spec.whatwg.org/#list-clone">https://infra.spec.whatwg.org/#list-clone</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-list-clone">4.2. Worker initialization</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-list-iterate">
   <a href="https://infra.spec.whatwg.org/#list-iterate">https://infra.spec.whatwg.org/#list-iterate</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-list-iterate">5. Request processing</a> <a href="#ref-for-list-iterate①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-map-key">
   <a href="https://infra.spec.whatwg.org/#map-key">https://infra.spec.whatwg.org/#map-key</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-map-key">3.2. Accept-CH cache</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-ordered-map">
   <a href="https://infra.spec.whatwg.org/#ordered-map">https://infra.spec.whatwg.org/#ordered-map</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-ordered-map">3.2. Accept-CH cache</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-ordered-set">
   <a href="https://infra.spec.whatwg.org/#ordered-set">https://infra.spec.whatwg.org/#ordered-set</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-ordered-set">3.1. Client hints set</a>
    <li><a href="#ref-for-ordered-set①">3.2. Accept-CH cache</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-map-set">
   <a href="https://infra.spec.whatwg.org/#map-set">https://infra.spec.whatwg.org/#map-set</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-map-set">3.2. Accept-CH cache</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-settings-object">
   <a href="https://w3c.github.io/webappsec-secure-contexts/#settings-object">https://w3c.github.io/webappsec-secure-contexts/#settings-object</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-settings-object">3.3. Initialize Client Hints set</a>
    <li><a href="#ref-for-settings-object①">3.4. Accept-CH state (http-equiv="accept-ch")</a>
   </ul>
  </aside>
  <h3 class="no-num no-ref heading settled" id="index-defined-elsewhere"><span class="content">Terms defined by reference</span><a class="self-link" href="#index-defined-elsewhere"></a></h3>
  <ul class="index">
   <li>
    <a data-link-type="biblio">[FEATURE-POLICY]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-default-allowlist" style="color:initial">default allowlist</span>
     <li><span class="dfn-paneled" id="term-for-policy-controlled-feature" style="color:initial">policy-controlled feature</span>
    </ul>
   <li>
    <a data-link-type="biblio">[FETCH]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-concept-header-list-append" style="color:initial">append</span>
     <li><span class="dfn-paneled" id="term-for-concept-request-client" style="color:initial">client</span>
     <li><span class="dfn-paneled" id="term-for-header-list-contains" style="color:initial">contains</span>
     <li><span class="dfn-paneled" id="term-for-header-list-contains①" style="color:initial">does not contain</span>
     <li><span class="dfn-paneled" id="term-for-concept-fetch" style="color:initial">fetch</span>
     <li><span class="dfn-paneled" id="term-for-concept-header" style="color:initial">header</span>
     <li><span class="dfn-paneled" id="term-for-concept-request-header-list" style="color:initial">header list</span>
     <li><span class="dfn-paneled" id="term-for-concept-http-redirect-fetch" style="color:initial">http-redirect fetch</span>
     <li><span class="dfn-paneled" id="term-for-concept-header-name" style="color:initial">name</span>
     <li><span class="dfn-paneled" id="term-for-navigation-request" style="color:initial">navigation request</span>
     <li><span class="dfn-paneled" id="term-for-concept-request-request" style="color:initial">request</span>
     <li><span class="dfn-paneled" id="term-for-subresource-request" style="color:initial">subresource request</span>
     <li><span class="dfn-paneled" id="term-for-concept-header-value" style="color:initial">value</span>
    </ul>
   <li>
    <a data-link-type="biblio">[HTML]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-attr-meta-content" style="color:initial">content</span>
     <li><span class="dfn-paneled" id="term-for-environment-settings-object" style="color:initial">environment settings object</span>
     <li><span class="dfn-paneled" id="term-for-the-head-element" style="color:initial">head</span>
     <li><span class="dfn-paneled" id="term-for-meta" style="color:initial">meta</span>
     <li><span class="dfn-paneled" id="term-for-concept-settings-object-origin" style="color:initial">origin <small>(for environment settings object)</small></span>
     <li><span class="dfn-paneled" id="term-for-relevant-settings-object" style="color:initial">relevant settings object</span>
     <li><span class="dfn-paneled" id="term-for-responsible-browsing-context" style="color:initial">responsible browsing context</span>
     <li><span class="dfn-paneled" id="term-for-same-origin" style="color:initial">same origin</span>
     <li><span class="dfn-paneled" id="term-for-top-level-browsing-context" style="color:initial">top-level browsing context</span>
    </ul>
   <li>
    <a data-link-type="biblio">[INFRA]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-set-append" style="color:initial">append</span>
     <li><span class="dfn-paneled" id="term-for-byte-lowercase" style="color:initial">byte-lowercase</span>
     <li><span class="dfn-paneled" id="term-for-list-clone" style="color:initial">clone</span>
     <li><span class="dfn-paneled" id="term-for-list-iterate" style="color:initial">for each</span>
     <li><span class="dfn-paneled" id="term-for-map-key" style="color:initial">key</span>
     <li><span class="dfn-paneled" id="term-for-ordered-map" style="color:initial">ordered map</span>
     <li><span class="dfn-paneled" id="term-for-ordered-set" style="color:initial">ordered set</span>
     <li><span class="dfn-paneled" id="term-for-map-set" style="color:initial">set <small>(for map)</small></span>
    </ul>
   <li>
    <a data-link-type="biblio">[secure-contexts]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-settings-object" style="color:initial">Is an environment settings object contextually secure?</span>
    </ul>
  </ul>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-client-hints">[CLIENT-HINTS]
   <dd>Ilya Grigorik; Yoav Weiss. <a href="https://httpwg.org/http-extensions/client-hints.html">Client Hints</a>. URL: <a href="https://httpwg.org/http-extensions/client-hints.html">https://httpwg.org/http-extensions/client-hints.html</a>
   <dt id="biblio-feature-policy">[FEATURE-POLICY]
   <dd>Ian Clelland. <a href="https://wicg.github.io/feature-policy/">Feature Policy</a>. URL: <a href="https://wicg.github.io/feature-policy/">https://wicg.github.io/feature-policy/</a>
   <dt id="biblio-fetch">[FETCH]
   <dd>Anne van Kesteren. <a href="https://fetch.spec.whatwg.org/">Fetch Standard</a>. Living Standard. URL: <a href="https://fetch.spec.whatwg.org/">https://fetch.spec.whatwg.org/</a>
   <dt id="biblio-html">[HTML]
   <dd>Anne van Kesteren; et al. <a href="https://html.spec.whatwg.org/multipage/">HTML Standard</a>. Living Standard. URL: <a href="https://html.spec.whatwg.org/multipage/">https://html.spec.whatwg.org/multipage/</a>
   <dt id="biblio-infra">[INFRA]
   <dd>Anne van Kesteren; Domenic Denicola. <a href="https://infra.spec.whatwg.org/">Infra Standard</a>. Living Standard. URL: <a href="https://infra.spec.whatwg.org/">https://infra.spec.whatwg.org/</a>
   <dt id="biblio-rfc2119">[RFC2119]
   <dd>S. Bradner. <a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>. March 1997. Best Current Practice. URL: <a href="https://tools.ietf.org/html/rfc2119">https://tools.ietf.org/html/rfc2119</a>
   <dt id="biblio-secure-contexts">[SECURE-CONTEXTS]
   <dd>Mike West. <a href="https://www.w3.org/TR/secure-contexts/">Secure Contexts</a>. 15 September 2016. CR. URL: <a href="https://www.w3.org/TR/secure-contexts/">https://www.w3.org/TR/secure-contexts/</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content">Issues Index</span><a class="self-link" href="#issues-index"></a></h2>
  <div style="counter-reset:issue">
   <div class="issue"> Define registry’s low entropy table<a href="#issue-f40790e6"> ↵ </a></div>
   <div class="issue">We need to figure out if we really want a <code>Sec-</code> prefix, and if so also exempt it from CORS.<a href="#issue-48a63eb6"> ↵ </a></div>
   <div class="issue"> Should Save-data really have an allowlist of <a class="property" data-link-type="propdesc">*</a>? If so, is it because it’s somehow "low entropy" and should we tie low-entropy-ness to allowlists, generally?<a href="#issue-ef477b6d"> ↵ </a></div>
   <div class="issue"> Links for image features are broken, need to actually define that and link to them.<a href="#issue-707decdc"> ↵ </a></div>
  </div>
  <aside class="dfn-panel" data-for="client-hints-set">
   <b><a href="#client-hints-set">#client-hints-set</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-client-hints-set">3.2. Accept-CH cache</a> <a href="#ref-for-client-hints-set①">(2)</a>
    <li><a href="#ref-for-client-hints-set②">4.5. Extending environment settings object</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="accept-ch-cache">
   <b><a href="#accept-ch-cache">#accept-ch-cache</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-accept-ch-cache">3.2. Accept-CH cache</a> <a href="#ref-for-accept-ch-cache①">(2)</a> <a href="#ref-for-accept-ch-cache②">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="accept-ch-cache-origin">
   <b><a href="#accept-ch-cache-origin">#accept-ch-cache-origin</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-accept-ch-cache-origin">3.2. Accept-CH cache</a> <a href="#ref-for-accept-ch-cache-origin①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="accept-ch-cache-client-hints-set">
   <b><a href="#accept-ch-cache-client-hints-set">#accept-ch-cache-client-hints-set</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-accept-ch-cache-client-hints-set">3.2. Accept-CH cache</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="add-a-new-accept-ch-cache-entry">
   <b><a href="#add-a-new-accept-ch-cache-entry">#add-a-new-accept-ch-cache-entry</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-add-a-new-accept-ch-cache-entry">3.3. Initialize Client Hints set</a>
    <li><a href="#ref-for-add-a-new-accept-ch-cache-entry①">3.4. Accept-CH state (http-equiv="accept-ch")</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="retrieve-the-client-hints-set">
   <b><a href="#retrieve-the-client-hints-set">#retrieve-the-client-hints-set</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-retrieve-the-client-hints-set">3.3. Initialize Client Hints set</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="abstract-opdef-initialize-the-client-hints-set">
   <b><a href="#abstract-opdef-initialize-the-client-hints-set">#abstract-opdef-initialize-the-client-hints-set</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-abstract-opdef-initialize-the-client-hints-set">4.1. Document object initialization</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="accept-ch-state">
   <b><a href="#accept-ch-state">#accept-ch-state</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-accept-ch-state">4.4. Pragma directives</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="accept-ch">
   <b><a href="#accept-ch">#accept-ch</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-accept-ch">3.4. Accept-CH state (http-equiv="accept-ch")</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="environment-settings-object-client-hints-set">
   <b><a href="#environment-settings-object-client-hints-set">#environment-settings-object-client-hints-set</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-environment-settings-object-client-hints-set">3.3. Initialize Client Hints set</a> <a href="#ref-for-environment-settings-object-client-hints-set①">(2)</a> <a href="#ref-for-environment-settings-object-client-hints-set②">(3)</a>
    <li><a href="#ref-for-environment-settings-object-client-hints-set③">3.4. Accept-CH state (http-equiv="accept-ch")</a> <a href="#ref-for-environment-settings-object-client-hints-set④">(2)</a> <a href="#ref-for-environment-settings-object-client-hints-set⑤">(3)</a>
    <li><a href="#ref-for-environment-settings-object-client-hints-set⑥">4.2. Worker initialization</a> <a href="#ref-for-environment-settings-object-client-hints-set⑦">(2)</a>
    <li><a href="#ref-for-environment-settings-object-client-hints-set⑧">5. Request processing</a> <a href="#ref-for-environment-settings-object-client-hints-set⑨">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="abstract-opdef-append-client-hints-to-request">
   <b><a href="#abstract-opdef-append-client-hints-to-request">#abstract-opdef-append-client-hints-to-request</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-abstract-opdef-append-client-hints-to-request">6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="abstract-opdef-remove-client-hints-from-redirect-if-needed">
   <b><a href="#abstract-opdef-remove-client-hints-from-redirect-if-needed">#abstract-opdef-remove-client-hints-from-redirect-if-needed</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-abstract-opdef-remove-client-hints-from-redirect-if-needed">6. Integration with Fetch</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="client-hints-token">
   <b><a href="#client-hints-token">#client-hints-token</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-client-hints-token">3.1. Client hints set</a>
    <li><a href="#ref-for-client-hints-token①">3.3. Initialize Client Hints set</a>
    <li><a href="#ref-for-client-hints-token②">3.4. Accept-CH state (http-equiv="accept-ch")</a> <a href="#ref-for-client-hints-token③">(2)</a>
    <li><a href="#ref-for-client-hints-token④">5. Request processing</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="low-entropy-hint-table">
   <b><a href="#low-entropy-hint-table">#low-entropy-hint-table</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-low-entropy-hint-table">5. Request processing</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="find-client-hint-value">
   <b><a href="#find-client-hint-value">#find-client-hint-value</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-find-client-hint-value">5. Request processing</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="field-name">
   <b><a href="#field-name">#field-name</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-field-name">3.3. Initialize Client Hints set</a>
    <li><a href="#ref-for-field-name①">3.4. Accept-CH state (http-equiv="accept-ch")</a>
   </ul>
  </aside>
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